pdfdoc.py: improve image masking given by Colors
pdfdoc.py: improve image masking given by Colors

kanarelo/reportlab,kanarelo/reportlab,Distrotech/reportlab,Distrotech/reportlab,Distrotech/reportlab,kanarelo/reportlab,kanarelo/reportlab,Distrotech/reportlab,kanarelo/reportlab,Distrotech/reportlab